The top federal prosecutor, D.C. U.S. Attorney Ronald C. Machen Jr., is quoted at a press conference as asserting the fact — before trial — that District of Columbia Mayor Vincent Gray knew about “businessman Jeffrey E. Thompson’s conspiracy to pump more than $600,000 in illegal donations into the campaign.”

In other words, a federal prosecutor, before indictment, before any evidence has been introduced at a trial, before a verdict, has pronounced Gray guilty of a serious crime at a press conference.

The mayor immediately responded: “These are lies.” He points out that Thompson, the key witness against Gray, is a convicted felon.

In today’s presumption-of-guilt culture, where headlines and accusation too often become surrogates for fact and truth, clearly Machen’s assertions and the headlines will have an adverse political effect on Gray. Another Post story quoted a D.C. resident named Caren Kirkland, for example, who said of Gray: “I feel like he sold the entire city out.”

Full disclosure: I work in the district (and live in Maryland) and have been a supporter of the mayor and think he has done a good job.

Given the headlines over the last four years implying he has committed various crimes associated with his 2010 election campaign, it is amazing to me that he has survived even to run for re-nomination.

From what I’ve read, he has a pretty good record: Unemployment is down over the last four years, from 11 percent to 7.6 percent; his budget has resulted in three straight years of surpluses; more than $180 million has been invested in affordable housing; public school students are showing significant improvements in urban area standard testing; crime rates have fallen; there are increased cops on the streets, etc.

The presumption of innocence — that every citizen is innocent until proven guilty in a court of law beyond a reasonable doubt — is a fundamental principle implicit in the due process clause of the 5th and 14th amendments of the U.S. Constitution. So the Supreme Court held in 1895.

The U.S. attorney is bound by that principle as much as everyone else in the criminal justice system.

Yet in today’s culture, many people actually believe — as Kirkland does — that where there is smoke (headlines of wrong-doing), there must be fire.

This isn’t the first time that I have written about my unease with prosecutors, the media and the public who ignore the presumption of innocence.

Last year a Fairfax, Va., teacher was accused of sexually molesting two teenagers. He was arrested and accompanied out of the school.

Headlines reported him as an accused child molester. He lost his teaching job, and he and his family were ostracized. Months later the two teenagers recanted and admitted they had lied about their charge.

I wrote a column at the time, regretting the unfair penalty this man was forced to pay by those who presume guilt before trial based on accusations alone. At that time, even after the charges were dropped, this innocent man still could not find a job.

In my memoir about my time as counsel in the Clinton White House, I wrote about the early 1980s when we Democrats in the U.S. Senate accused President Reagan’s secretary of labor, Raymond Donovan, of organized crime connections.

Democrats succeeded in getting three independent counsel investigations, and each time no crime was found. He was forced to resign in disgrace.

Then he was indicted in the Bronx, N.Y. Two years later, he went to trial. The jury acquitted him in less than 10 hours. After the verdict, he turned to one of the prosecutors and asked the now famous, and painful, question: “Which office do I go to get my reputation back?”

As a partisan Democrat back then, I cheered on the hounding and prosecution of Donovan.

There are no entirely clean hands here. Certainly not mine.

Mayor Vincent Gray deserves the presumption of innocence. It is only fair. It is — it must be — the American way.

This column appears first and weekly in The Hill and the Hill.com.

Lanny Davis is the principal in the Washington, D.C., law firm of Lanny J. Davis & Associates, which specializes in strategic crisis management. He served as President Clinton’s Special Counsel in 1996-98. Read more reports from Lanny Davis — Click Here Now.